Apparatus for heating metals to high temperatures



July 1965 E. NAVEZ ETAL APPARATUS FOR HEATING METALS TO HIGHTEMPERATURES 4 Sheets-Sheet 1 Filed July 25, 1962 y 27, 1955 NAVEZ ETAL3,197,184

APPARATUS FOR HEATING METALS TO HIGH TEMPERATURES Filed July 25, 1962 4Sheets-Sheet 2 Fig 2 Fig 3 Fig 5 4 Sheets-Sheet 5 E. NAVEZ ETALAPPARATUS FOR HEATING METALS TO HIGH TEMPERATURES July 27, 1965 FiledJuly 25, 1962 y 7, 1965 E. NAVEZ ETAL 3,197,184

APPARATUS FOR HEATING METALS TO HIGH TEMPERATURES Filed July 25, 1962 4Sheets-Sheet 4 United States Patent 3,197,184 APPARATUS FQR Z EATWGMETALS TO HIGH TEMPERATURES Emile Navez and (Camille Demoulin, Paris,France, as-

siguors to Stein & Roubaix, Paris, France, a company of France FiledJuly 25, 1962, der. No. 212,279 Claims prior-2W, application France,Nov. 13, 1% 878,797 6 Claims. (Cl. 2637) In many manufacturing processesinvolving large de formations of the treated metal, particularly in thecase of steel, it is necessary to raise the metal to a high temperature,which may attain as much as 1250 to 1300" C.

Conventional heating methods using oxidizing combustion lead tooxidation of the metal, and the scale resulting from the oxidationcauses large losses of metal due to the heating, along with rapiddeterioration of the conversion tooling used on the heated metal.

From studies made on the subject, it is well known that to ensureoxidation-free heating of steel to high temperatures, the metal mustevolve between 650/70() C. and 1250/ 1300 C. in an atmosphere thequality of which is defined by the following approximate ratios:

Such an atmosphere can only be obtained by the incomplete combustion(hereinafter termed primary combustion) of a gaseous fuel, using roughly50% of the amount of air theoretically required for complete combustion.

Combustion with such a shortage of air will not give off all the latentheat of the fuel and will take place at temperatures that are low enoughto prevent completion of the combustion and dissociation reactions. Thisresults in the obtainment of oxidoreducing atmospheres, in conjunctionwith lampblack deposits on the products to be heated. This latterphenomenon is encountered particularly in the case of the incompletecombustion of rich gaseous fuels composed mainly of hydrocarbons whichhave escaped controlled cracking at some localized point.

in order to overcome these drawbacks of inadequate temperature forheating the product, of oxidoreducing combustion, and of lampblackdeposits caused by disorderly cracking, a variety of solutions have beenput forward.

It has been proposed that the combustion be effected in tWo stages, oreven in three stages; industrial designs to conform with such proposals,however, are not applicable to conventional furnace designs, or raiseextremely tricky technological problems in connection with theresistance to fire of the refractory materials employed. Moreover, theartifice of effecting combustion in two stages in the same heatingchamber means certain pollution of the reducing atmosphere.

Another proposal calls for recourse to reversal processes for obtainingan adequate preheating of the air, so as to be able to attain hightemperatures with reducing combustions. This heating method, however,has all the inherent drawbacks of the reversal system, namelyinconsistency of the required atmosphere due to leaks of gas and airinto the smoke during the reversals.

This invention has for its object to provide an industrial furnace ofconventional design, capable of providing continuous oxidation-frcemetal heating up to temperatures of 1250 to 1300 C., for example.

To this end, and in accordance with the present invention, the metal isheated to a high temperature by means of a direct flame resulting fromthe incomplete combus- 39 i 9 1 Patented July 27, 1965 tion of a gaseousfuel with combustive air, both the gas and the air being heatedcontinuously in a recovery system utilizing the sensible heat of theneutral or oxidizing smoke issuing from. the furnace.

The zone providing heating in a constant reducing atmosphere basicallycomprises solid masonry-work made of the usual refractory materials anddevoid of openings and absolutely lealrproot, evacuation of thecombustible smoke resulting from the primary reducing combustion takingplace transversely at one extremity of said zone, as in an ordinaryfurnace, instead of through apertures provided in the uprights, thehearth or the arch.

The primary combustion system is incorporated in the uprights andconsists of burners designed to give rapid and energetic combustion ofthe hot air and gas supplied to them under pressure after beingpreheated in recuperators that are entirely external to the furnace,i.e. separate therefrom. It is to be noted that the device for heatingthis zone does not provide for contributory heat, through the uprights,the hearth and the arch, derived from a secondary combustion of thecombustible products stemming from the primary combustion.

The distribution of hot air and gas to said furnace through a knownarrangement of lagged piping of customary design, in conjunction withthe burner design, enables high temperatures to be obtained which ensurerapid combustion and dissociation reactions and which produce a constantreducing atmosphere immediately upon exit from the burners andthroughout the gaseous mass occupying the volume of the oxidation-freeheating zone.

The sensible heat contributed by the fuel and the oxidant during themixing at the burner nozzles compensates for the calorific loss due tothe endothermic reaction which the dissociation of hydrocarbons, so thatthe desired physico-chemical balance is finally achieved without anyfear of an oxidoreducing mixture.

On issuing from the oxidation-free heating zone, the combustible smokeis conveyed by natural draught to a metal-preheating zone whichcommunicates with the oxidation-free heating zone.

Within said preheating zone, a portion of the combustible productsoriginating from the oxidation-free heating zone is caused to sustain asecondary afterburning process, in order to heat the products disposedupon the furnace hearth. The secondary afterburner is so designed thatair atmosphere which is still reducing be maintained in said preheatingzone, to ensure that the products to be heated are in no case exposed toan oxidizing atmosphere right from the point where they are charged intothe furnace up to their point of entry into the oxidation-free heatingzone.

The smoke issuing from the heating zone still contains combustibleconstituents and is directed, by a natural draught process, into a flueincorporating a system for providing tertiary afterburning of the smokestill containing combustible constituents, as the result of whichcompletely burnt-out combustion products can be discharged into suitablerecuperators and thence out through the chimney.

Said tertiary afterburner provides the possibility of diluting thecompletely burnt-out smoke by injecting additional air, in order toavoid circulating unduly hot smoke through the fiue.

With a view to ensuring perfectly uniform secondary and tertiaryafterburning (for instance when a secondary afterburning is used inorder to avoid the products to be heated becoming oxidized because ofimperfect mixing), it will be of advantage for the total secondary andtertiary combustive air to be preheated to a lower temperature than thatof the primary combustible air.

Further particularities and advantages of the present "ice J) inventionwill become apparent from the description which follows of a specificembodiment of the invention, given with reference to the accompanyingdrawing, which is filed by way of example and not of limitation.

In the drawing filed herewith:

FIGURE 1 is a sectional view, taken through a plane parallel to thehearth, of a furnace executed in accordance with the invention;

FIGURE 2 is a sectional view of the same furnace, taken through thelines II-II of FIGURE 1;

FIGURES 3 and 4 are further sectional views of said furnace, takenthrough the lines III- 1H and IV-1V, respectively, in FIGURE 1;

FIGURE 5 is a vertical sectional view of a tertiary afterburning device;

FIGURE 6 is a longitudinal sectional view, on a larger scale, of aprimary-combustion reducing burner; and

FIGURE 7 is a diagram showing the manner of regulation of the furnace inFIGURE 1.

Referring now to the accompanying drawing, the furnace described withreference thereto is a rotary hearth furnace of conventional designcapable of ensuring, in continuous production, the oxidation-freeheating of billets, ingots or other steel workpieces, up to temperaturesof around l250 to 1300" C., for example.

Reference is now had to FIGURE 1 in particular, wherein it will be seenthat the rotary hearth furnace illustrated comprises six zones A to Fbounded by six radial barriers 1 to 6. The number of Zones is by nomeans critical and has here been given by way of example only and not oflimitation.

The products are introduced into zone A through a charging door 7, aredrawn along by the rotary hearth 9 in the direction of the arrow G andare evacuated from zone F through the discharging door 8.

The furnace is of the counter-streaming type, is. the smoke travels inthe direction opposite to the direction of rotation of the products.

That part of the furnace in which the heavily reducing atmosphere isobtained by incomplete primary combustion and in which the metal isheated over the range which is dangerous from the oxidation standpoint,comprises four zones, of which only three are equipped with burnersproducing the reducing combustion. Starting from the discharging door 8,said four zones are the following:

One equalization zone F, comprised between barriers '6 and 5 andreducto-combustion-he-ated by burners If) such as those describedhereinbefore with reference to FIG- URE 6.

Two heating zones E and D, respectively included between barriers 5 and4 and 3, and likewise reducto-corn bustion-heated by burners 10, such asthe burner shown in FIGURE 6.

One heating zone'C devoid of burners, comprised between barriers 3 and 2and in wh ch the reducing combustion products from the preceding zoneslose part of their sensible heat, thereby heating the metal while at thesame time protecting it against oxidation.

Said four zones have the smallest possible transverse dimensions, as isclearly shown in FIGURE 2, the object of this being to obtain a veryhigh calorific density per unit volume of internal zone-space.

Such a disposition makes it possible to achieve high temperatures andhightly efficient heat transfers due to the proximity of the products tobe heated to the flames issuing from burners which have beenspecifically designed to give very bright flames.

Near the end of zone C (with reference to the direction of smokecirculation), part of the primary combustion products is sucked throughorifices 11:: provided in the furnace uprights on a level with thehearth 9 and is led through fiues 11b to the first secondaryafterburning burners 12 in preheating zone B, while the remaining partof the primary combustion products passes beneath barrier 2 and abovethe products being treated (see FIGURE 3).

of the masonry forming the uprights 11.

Thus the blanket of protective atmosphere which passes beneath barrier 2provides protection for the metal products over the temperature rangecorresponding to incipient oxidation, while at the same time supplyingheat to the metal.

Another part of the primary combustion products is drawn throughorifices 13 provided in the uprights 11 level with hearth 9, and is ledthrough flues 14 to other secondary afterburning burners 12 (see FIG.4).

The rising motion of this part of the primary combustion productsthrough the flues 14 is created by the induction of hot secondarycombustion air delivered under pressure to the burners 12 through pipes15 and tubes 16. Thus preheating zone B is used to burn the quantitiesof primary combustion product required to heat the metal to thetemperature of entry into the oxidation-free heating zone C andcompensate for the calorific losses.

The products resulting from this secondary afterburning are evacuatedfrom the furnace through vertical lines I and led into the combustionchambers 18, in which tertiary afterburning is then effected with hotair (see FIGURE 5). This latter afterburning is performed with hot airdelivered under pressure into rings 19 and introduced into chamber 18via inclined passageways 19a.

Since this tertiary afterburning process can result in a large quantityof heat being given off in certain cases thereby leading to high smoketemperatures in the fines 17a connected to the main fine 41 provision ismade for cold diluting air delivered through pipes 20 and tubes 21.Subsequent to the tertiary afterburning and to dilution of the smoke,the latter is conveyed into the chimney 45 through the flue 41, whichflue incorporates recuperators such as those designated by referencenumerals 4t 43 and 44.

The zones having a heavily reducing atmosphere, i.e. the equalizationand heating zones F, E and D comprised between barriers 6 and 3, arepreferably equipped with burners such as the burner illustrated inFIGURE 6.

Such a burner is supplied with hot air and gas at the temperaturesrequired to ensure reductive combustion devoid of oxygen and lampblack,together with a high ambient temperature.

The hot gas is delivered under pressure through a lagged pipe 22 leadingfrom recuperator 40. The pipe 22 is connected to a gas tube 23 by meansof a flange 24. The flange 24 is welded to tube 23 and is renderedco-extensive with a certain length thereof by means of threaded sleeve25. Sealing is provided by means of a stuffing-box 26 which is screwedto sleeve 25 and which clamps a packing seal against a socket 25a intowhich the sleeve 25 rigid with gas tube 23 is screwed.

The gas tube is free to rotate, advance or withdraw to permit adjustmentand can be restrained in the desired position by a locknut 27. T o theextremity of the tube is welded a refractory nozzle 28, and the gasissues therefrom through holes 29 spaced regularly around the outersurface of the nozzle. In the specific embodiment illustrated, thenozzle 23 is provided with a convergent duct, followed by a cylindricalduct which is joined to the holes 2? through a divergent duct. The axesof the holes 29 are splayed about the longitudinal axis of tube 23 andthe diameter of the holes 29 is chosen so that the gas velocity is veryhigh.

The hot air from recuperator 43 is also delivered under pressure througha pipe 30, which pipe is connected to a T-tube 31 one of the branches ofwhich surrounds tube 23 and is rigid with socket 25a. Sealing withrespect to the furnace is achieved by means of a flange 32 which isrigid with tube 31 and with joints 33 affixed to the furnace enclosure34. Lagging 32a surrounds all the piping outside the furnace.

Tube 31 debouches into a special part made of refractory concrete andcomprised within the furnace masonrywork. The part 35 extends throughthe entire thickness Over nozzle 28 is fitted a moulded part 36 made ofhighly refractory material ports 37 which are so disposed that theiraxes be at right angles to those of the gas holes 29. Air under pressuredelivered through tube 31 into the space comprised between parts 35 and36 debouches at high velocity through ports 37 and impinges at rightangles upon the gas fillets issuing from holes 29. The conical section37a is extended by a conical part 33 which preferably embodies spiralgrooves or spiral ribs.

An energetic combustion is thus initiated in the cone 37a of part 35,and this combustion takes place with a high degree of turbulence,extends through part 38 (the grooves or ribs of which further accentuatethe turbulence), and terminates perpendicularly in line with theupright. This rapid and energetic combustion allows for dissociationreactions of short duration, thereby leading to physico-chemicalequilibrium of exit from the burners, without any trace of oxygen anddevoid of lampblack.

The cracking which the hydrocarbons undergo is endothermic, and thepreheating of the gas compensates for the calorific losses due to thedissociation reactions of the fuel. Thus, by virtue of the turbulence,the heating of the gas, the heating of the air, the disposition of theair and gas orifices, and the high discharge velocities of the air andthe gas, a very energetic combustion is ensured, which results in veryhigh pyrometric efliciency. The eminently luminous flame obtained withthis device is an excellent heat transmission factor.

Since all the combustion and dissociation reactions are completed inproximity to the burner, by reason of the high temperature and theturbulence, they ensure stable atmospheres within the furnace.

The method whereby hot gas and hot air are supplied to the furnace isillustrated diagrammatically in FIG- URE 7.

The oxidation-free heating zones D, E, F being the only ones to besupplied with gas, the gas distribution circuit comprises only one pipeleading to the furnace, which pipe terminates in the various branchessupplying the burners.

The hot air distribution system may be divided into three circuits:

The first for supplying burners in oxidation free heating zones D, E andF.

The second for supplying burners 12 in the secondary afterburning zoneB.

The third for supplying the burners in tertiary afterburning chamber 18.

Regulation of these supply circuits is so accomplished that with the gasas the pilot fluid and a regulating flowrneter measuring the same, asecond flowmeter measures and regulates the total combustion airsupplied to the furnace. The air flow regulator is slaved to the gasflow regulator through the medium of a ratio adjuster which enables theair to be metered in terms of the gas fiow and a complete neutralcombustion to be invariably ensured after the tertiary afterburningprocess.

In addition, the proportionality existing between the gas and the air ateach individual burner of each set of burners in the omdation-freeheating zones is ensured by a ratio adjuster, which makes it possible toobtain reducing atmospheres of constant composition regardless of theflow rates.

The cold gas arriving from a distribution station through a duct 39 isheated in recuperator 4%, which is located in the smoke flue 41extending between the furnace and chimney 45.

The total combustion air is supplied by a single blower 4-2; it isheated in the two recuperators 43 and 44, recuperator 43 serving for theprimary combustion air and recuperator 44 for the secondary and tertiaryafterburning air. The recuperators 53 and 44 are positioned upstream ofgas-heating recuperator 4%}, the word upstream being 6 used withreference to the direction of evacuation of the smoke towards thechimney.

Inserted into the cold gas circuit is a diaphragm 46 connected to aregulating fiowmeter 47. Similarly inserted into the cold air circuit isa diaphragm 48 and a regulating flowmeter 49. The gas and air flowmeters47 and 49 are interconnected by a ratio adjuster 50 which maintains theair-to-gas ratio constant at the figure corresponding to neutral orslightly oxidizing combustion, this being achieved by means of a powervalve 51 which regulates the cold air flow. This provides a certainty ofcompletely burnt smoke subsequent to the tertiary afterburning process.

The equalization zone F and the heating zones E and D are supplied withhot air through the pipes 30 (drawn in solid lines) connected to thepipes 53, which pipes are in turn connected to a pipe 53a leading fromrecuperator 43. They are supplied with hot gas through pipes 22 (shownin broken lines on the drawing) connected to pipes 52, which pipes 52are in turn connected to a main duct 52a leading from recuperator 46 Thegas flows are adjusted by power valves 54 and the air flows by similarvalves 55. The gas valves 54- are controlled by regulating fiowmeters 56and the air valves 55 by regulating flowmeters 57. The gas and airflowmeters 56 and 57 are interconnected by ratio adjusters 58 whichmaintain the air-to-gas ratios constant in the zones having highlyreducing atmospheres, and this is achieved independently in each zone.The gas and air flowmeters 56 and 57 are connected to a temperatureregulator 59 which is in turn connected to a temperature detector 60.When the temperature increases in the equalization zone for instance,regulator 59 of zone F reduces the gas and air flows by means ofregulating flowmeters 56 and 57 and power valves 54 and 55, and 'at thesame time maintains an invariable atmosphere in that zone by means ofthe associated ratio adjuster 58.

In preheating zone B, a temperature detector 61 is connected to aregulator 62 which controls a power valve 63. The power valve 63regulates the flow of hot air through the duct 63a which extends betweenrecuperator 44 and the pipes 15 of burners 12. If the temperatureincreases in the preheating zone for example, valve 23 closes and, sincethe air how is the same in recuperator 44, the air which no longersupplies preheating zone B feeds the tertiary combustion process throughthe duct 63b which leads up to rings 19 of combustion chambers 18. Theflow of secondary air is regulated by a flowmeter 64 and the flow oftertiary air by a flowmeter 65.

The dilution air which cools the smoke subsequent to the tertiarycombustion and which is introduced into flues 17:; through pipes 25 andtubes 21 is supplied by an independent blower 66. A temperature detector67 is connected to a temperature regulator 68 which controls a fiowmeter69, which fiowmeter in turn controls a power valve for regulating theflow of dilution air. Temperature detector 57 can .be positioned withinthe smoke, ahead of the first recuperator 44, or alternatively withinthe primary air exiting from its recuperator.

It will of course be understood that, without depart ing from the scopeof the invention, many modifications and substitutions of parts may bemade to the specific embodiment hereinbefore. described with referenceto the accompanying drawings.

What we claim is:

1. A continuously operating furnace installation for heating metallicwork on a hearth to high temperatures Without oxidation of the Work,comprising:

a hearth;

a first heating zone having an outlet opening for the discharge of work,and burner means directed toward the work;

means providing said burner means with preheated rich gaseous fuel andpreheated primary air in such ratio as to produce incomplete com- 7bustion products establishing a reducing atmosphere in the first heatingzone;

at least one other heating zone separated from the first heating zone bybarrier means located at such a distance above the hearth as to allowthe passage of Work and of part of said incomplete combustion products,said other heating zone being provided with inlet means for theadmission of Work,

afterburner means in the atmosphere thereof; means for feeding some ofsaid incomplete combustion products to said afterburner means;

means for providing said afterburner means with preheated secondary airin such ratio with the incomplete combustion products supplied theretoas to produce partial afterburning of said incomplete combustionproducts, giving thereby reducing smoke in said other heating zone;

flue means leading from the other zone providing a draught of theincomplete combustion products from the first-named heating zone throughthe other heating zone and out of said other heating zone,

the first zone and said other heating zone thereby providingcounter-current heating of the work Without oxidation thereof; andoutside the furnace proper a tertiary afterburning chamber connected tosaid flue means and provided with teritary afterburner means;

means for providing said tertiary afterburner means with teritarypreheated air in such a ratio as to cause a complete combustion of saidreducing smoke into burnt smoke;

conduit means connecting said tertiary afterburning chamber to achimney;

means for delivering cold diluting air into said conduit means;

a gas recuperator mounted on said conduit means in heat-exchangerelationship with' said burnt smoke for preheating said gaseous fuel;

and at least one air recuperator arranged on said conduit means inheat-exchange relationship with said burnt smoke for preheating saidprimary combustion air and said secondary and teritary afterburning air.

2. The combination claimed in claim 1, the means for providing saidburner means with preheated rich gaseous fuel and preheated primary aircomprising pipe means for conducting said preheated air and preheatedrich gaseous fuel to said burner means, said burner means having amixing chamber opening into the interior of the first-named zone, andmixing means for discharging said preheated air and said preheated richgaseous fuel separately into said mixing chamber.

3. A furnace according to claim 2 wherein said pipe means comprise fueltubes and air passages, and said mixing means comprise ports connectingsaid air passages and said mixing chamber and holes connecting said fueltubes and said mixing chamber, said holes inclined with respect to theaxis of said fuel tubes, said ports being directed in a directionsubstantially perpendicular to the axes of said fuel tubes whereby saidpreheated air issues from said ports into said mixing chamber in adirection substantially perpendicular to said preheated fuel issuingfrom said holes.

4. A continuously operating counter-streaming furnace for heatingmetallic work to high temperatures without oxidation, comprising anequalization zone provided with burner means,

at least one primary combustion zone separated from said equalizationzone by a barrier and provided with primary burner means,

at least one heating zone devoid of burners, separated from said primaryzone by a barrier and receiving the combustion products therefrom,

at least one preheating zone separated from said heating zone by abarrier and provided with secondary burner means providing a flame inthe atmosphere of said preheating zone,

duct means for feeding said secondary burner means with a portion ofsaid combustion products from said primary zone,

at least one teritary combustion chamber provided with tertiary burnermeans,

fiues connecting said preheating zone to said tertiary combustionchamber,

feeding means to introduce combustion products from said preheating zoneinto said chamber,

a first recovery system fed with the smoke from said tertiary combustionchamber,

means to feed said first recovery system with primary combustion air andwith gaseous fuel in heat-exchange relationship with said smoke in saidsystem thereby to preheat said air and fuel,

pipe means for leading separately said preheated air and fuel to saidprimary burner means,

means for proportioning the ratio of air to fuel to said primary burnermeans in such ratio as to provide incomplete burning of the fuel,

mixing means to provide intimate mixture of said preheated air and fuelimmediately beyond said primary burner means thereby to produce anincomplete, rapid and energetic primary combustion devoid of oxidationin said primary combustion zone,

a second recovery systemv in series with said first recovery system,

means for feeding said second recovery system with secondary andtertiary combustion air in heat-exchange relationship with said smoke insaid second recovery system, and

leans for leading said preheated secondary and tertiary preheated air tosaid secondary and tertiary burner means respectively.

5. A continuously operating furnace installation for heating metallicwork on a hearth to high temperatures without oxidation, said furnaceinstallation having a work outlet and a work inlet and comprising:

a rotary hearth;

a first zone comprising first series of sub-zones starting with a firstsub-zone having said work outlet; and another zone comprising a secondseries of sub-zones continuing from the last sub-zone of the firstseries and terminating in a last sub-zone having said work inlet;

all of said sub-zones, save for the last sub-zone of the second seriesand the next adjacent one being separated one from the other by barriermeans located at such a distance above the hearth as to allow thepassage there-beneath of work and incomplete combustion products;

said last sub-zone of said second series and said next adjacent sub-zonebeing separated from one another by a barrier located at such a distanceabove the hearth as to permit the passage therebeneath of work whilesubstantially excluding the passage therebeneath of incompletecombustion products;

each of said sub-zones of said first series having therein burner meansdirected towards the work;

means providing the aforesaid burner means with preheated rich gaseousfuel and preheated primary air in such ratio as to produce incompletecombustion products establishing a reducing atmosphere in each of saidfirst series of sub-zones;

afterburner means in the atmosphere of a certain one of the sub-zones ofsaid second series other than said last one;

means for supplying to said afterburner means incomplete combustionproducts from a certain other sub-zone which is disposed on the side ofsaid certain one of the sub-zones which is remote from said lastsub-zone of said second series;

means for providing said afterburner means with preheated secondary airin such ratio with the incomplete combustion products supplied theretoas to produce partial afterburning of said incomplete combustionproducts, giving thereby reducing smoke in the sub-zone containing theafterburner means; and flue means leading from the sub-zone containingthe afterburner means, said flue means providing a draught of theincomplete combustion products from the first sub-zone of the firstseries through the other sub-zones save for the last one of the secondseries, thereby providing counter-current heating of the work in areducing atmosphere; there being a sub-zone of said second series whichis disposed between the last sub-zone of the first series and saidcertain one sub-zone of the second series which is devoid of burnermeans and atterburner means. 6. A furnace according to claim 5, saidatterburner means being disposed above the hearth level, the means 10for feeding some of said incomplete combustion products to saidafterbumer means comprising conduit means leading to said afterburnermeans from adjacent hearth level from the sub-zone which is devoid ofburner means and afterburner means.

References Cited by the Examiner UNITED STATES PATENTS 2,233,474 6/38Drefiein 263-15 2,639,910 11/49 Cone et al. 2665 2,622,863 5/50 Dauch263-28 2,799,491 12/54 Rusciano 263-15 2,845,260 7/58 Rusciano 266-53,022,057 10/59 Schmidt et al. 26315 3,106,192 10/63 Hingst 1227 FOREIGNPATENTS 680,057 -8/ 39 Germany.

CHARLES SUKALO, Primary Examiner.

1. A CONTINUOUSLY OPERATING FURNACE INSTALLATION FFOR HEATING METALLICWORK ON A HEARTH TO HIGH TEMMPERATURES WITHOUT OXIDATION OF THE WORK,COMPRISING: A HEARTH; A FIRST HEATING ZONE HAVING AN OUTLET OPENING FORTHE DISCHARGE OF WORK, ANND BURNER MEANS DIRECTED TOWARD THE WORK; MEANSPROVIDING SAID BURNER MEANS WITH PREHEATED RICH GASEOUS FUEL ANDPREHEATED PRIMARY AIR IN SUCH RATIO AS TO PRODUCE INCOMPLETE COMBUSTIONPRODUCTS ESTABLISHING A REDUCING ATMOSPHERE IN THE FIRST HEATING ZONE;AT LEAST ONE OTHER HEATING ZONE SEPARATED FROM THE FIRST HEATING ZONE BYBARRIER MEANS LOCATED AT SUCH A DISTANCE ABOVE THE HEARTH AS TO ALLOWTHE PASSAGE OF WORK AND OF PART OF SAID INCOMPLETE COMBUSTION PRODUCTS,SAID OTHER HEATING ZONE BEING PROVIDED WITH INLET MEANS FOR THEADMISSION OF WORK, AFTERBURNER MEANS IN THE ATMOSPHERE THEREOF; MEANSFOR FEEDING SOME OF SAID INCOMPLETE COMBUSTION PRODUCTS TO SAIDAFTERBURNER MEANS; MEANS FOR PROVIDING SAID AFTERBURNER MEANS; MEANS FORPROVIDING SAID AFTERBURNER MEANS WITHH PREHEATED SECONDARY AIR IN SUCHRATIO WITH THE INCOMPLETE COMBUSION PRODUCTS SUPPLIED THERETO AS TOPRODUCE PARTIAL AFTERBURNING OF SAID INCOMPLETE COMBUSTION PRODUCTS,GIVING THEREBY REDUCING SMOKE IN SAID OTHER HEATING ZONE; FLUE MEANSLEADING FROM THE OTHER ZONE PROVIDING A DRAUGHT OF THE INCOMPLETECOMBUSTION PRODUCTS FROM THE FIRST-NAMED HEATING ZONE THROUGH THE OTHERHEATING ZONE AND OUT OF SAID OTHER HEATING ZONE, THE FIRST ZONE AND SAIDOTHER HEATING ZONE THEREBY PROVIDING COUNTER-CURRENT HEATING OF THE WORKWITHOUT OXIDATIONN THEREOFF; AND OUTSIDE THE FURNACE PROPER A TERTIARYAFTERBURNING CHAMBER CONNECTED TO SAID FLUE MEANS AND PROVIDED WITHTERITARY AFTERBURNER MEANS; MEANS FOR PROVIDING SAID TERTTIARYAFTERBURNER MEANNS WITH TERITARY PREHEATED AIR IN SUCH A RATIO AS TOCAUSE A COMPLETE COMBUSTION OF SAID REDUCING SMOKE INTO BURNT SMOKE;CONDUIT MEANS CONNECTING SAID TERTIARY AFTERBURNING CHAMBER TO ACHIMNEY; MEANS FOR DELIVERING COLD DILUTING AIR INTO SAID CONDUIT MEANS;A GAS RECUPERATOR MOUNTED ON SAID CONDUIT MEANS IN HEAT-EXCHANGERELATIONSHIP WITH SAID BURNT SMOKE FOR PREHEATING SAID GASEOUS FUEL; ANDAT LEAST ONE AIR RECUPERATOR ARRANGED ON SAID CONDUIT MEANS INHEAT-EXCHANGE RELATIONSHIP WITH SAID BURNT SMOKE FOR PREHEATING SAIDPRIMARY COMBUSTION AIR AND SAID SECONDARY AND TERITARY AFTERBURNING AIR.